Estimation of water cloud properties from satellite microwave, infrared and visible measurements in oceanic environments 1. Microwave brightness temperature simulations

Theoretical calculations are used to examine the spectral characteristics of SSM/I (special sensor microwave/imager) brightness temperature (Tb) values for non-precipitating clouds over oceans. It was found that liquid water path (LWP) and the cloud water temperature (Tw) could be derived simultaneously with a technique using the SSM/I 37-GHz and 85-GHz brightness temperatures. Uncertainties in column water vapor (CWV) are the most important error sources in the estimation of LWP and Tw, while ice particles smaller than 100 mu m in nonprecipitating clouds have a very weak effect (< 1 K) on the Tb values at the relevant SSM/I frequencies. When all SSM/I instrument noise and error sources associated with sea surface temperature, wind speed, and CWV are considered, the biases in LWP from current microwave methods are very small (less than or equal to 0.01 mm) and the standard deviations vary from 0.02 to 0.04 mm. The Tw bias and standard deviation decrease with increasing LWP from about 6 and 8 K, respectively, for clouds with low LWP to < 1 K for LWP > 0.4 mm. For most marine stratocumulus clouds (LWP similar to 0.1 to 0.2 mm) the Tw bias and standard deviation are about 2 and 4 K, respectively, resulting in cloud height errors of similar to 1 to 2 km. The method should yield an improvement in the accuracy of retrieved LWP because it more closely approximates cloud temperature than previous techniques. To use the radiative transfer results, it is necessary to normalize or calibrate them to the observations. This relative calibration using 22-GHz brightness temperatures reveals differences of 2.86 K and -1.93 K for the 37-GHz horizontal and 85-GHz vertical channels, respectively, between the SSM/I observations and the model simulations. In multilayered cloud conditions, this new microwave analysis method, when combined with infrared data should make it possible to determine cloud temperature for an upperlevel ice cloud from the infrared brightness temperatures while simultaneously deriving Tw and LWP for the lower liquid water cloud with the microwave data.